Optical fiber connectors are a critical part of essentially all optical fiber communication systems. For instance, such connectors are used to join segments of fiber into longer lengths, to connect fiber to active devices (e.g., radiation sources, detectors and repeaters), and to connect fiber to passive devices (e.g., switches, multiplexers, and attenuators). A typical optical fiber connector comprises a housing and a ferrule within the housing. The ferrule has one or more bore holes, and a fiber secured in each bore hole such that the end of the fiber is presented for optical coupling by the ferrule. The housing is designed to engage a “mating structure” having an optical path to which the fiber optically couples during mating. The mating structure may be another connector or an active or passive device as mentioned above. The optical path may be, for example, a fiber in a ferrule, a waveguide in a substrate, a lens, or an optically-transparent mass. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially aligned with the optical pathway of the mating structure. This way, light from the fiber is optically coupled to the optical pathway.
Of particular interest herein are “expanded beam” optical connectors. Such connectors are used traditionally in high vibration and/or dirty environments, where “physical contact” between the fiber and the light path of mating connector is problematic. Specifically, in dirty environments, particulates may become trapped between connectors during mating. Such debris has a profoundly detrimental effect on the optical transmission since the particles are relatively large compared to the optical path (e.g., 10 microns diameter in single mode) and are therefore likely to block at least a portion of the optical transmission. Furthermore, in high-vibration environments, optical connectors having ferrules in physical contact tend to experience scratching at their interface. This scratching diminishes the finish of the fiber endface, thereby increasing reflective loss and scattering.
To avoid problems of debris and vibration, a connector has been developed which expands the optical beam and transmits it over an air gap between the connectors. By expanding the beam, its relative size increases with respect to the debris, making it less susceptible to interference. Further, transmitting the beam over an air gap eliminates component-to-component wear, thereby increasing the connector's endurance to vibration. Over the years, the expanded beam connector has evolved into a ruggedized multi-fiber connector comprising an outer housing, which is configured to mate with the outer housing of a mating connector, typically through a screw connection. Contained within the outer housing are a number of inner assemblies or “inserts.” Each insert comprises an insert housing, a ferrule assembly contained within the insert housing and adapted to receive a fiber, and a ball lens at a mating end of the insert housing optically connected to the fiber. The ball lens serves to expand and collimate light at the connector interface. When two expanded beam connectors are mated, there is an air gap between the ball lenses of each pair of optically coupled inserts.
Tyco Electronics Corporation (Harrisburg, Pa.) currently offers a line of expanded beam connectors under the brand name PRO BEAM®. Referring to FIGS. 4(a) and (b), the single mode and multimode PRO BEAM connector inserts 41, 42 are shown schematically. The single mode (SM) expanded beam connector 41 uses a PC-polished ferrule 43 that is in contact with a glass ball lens 44. (Note: a Physical Contact (PC) polish is slightly rounded, and the surface of the fiber is nominally perpendicular to the fiber axis. A flat-polished ferrule can also be used for single mode with good results because the relatively small radius of the lens will still achieve PC-contact with the fiber endface. See, for example, Telcordia GR-326.) The lens 44 is AR coated on one side for a glass/glass interface, and, on the other side, for an air/glass interface. The multimode (MM) connector 42 of FIG. 4(b) uses a flat-polished ferrule 45, which is held, at a fixed distance from the ball lens 46 by means of a stop or a spacer 47 that is located near the ball lens. The ball lens has an antireflective (AR) coating 48 for an air/glass interface to reduce Fresnel losses. Although the multimode and single mode expanded beam connectors offered by Tyco Electronics have consistently met industry requirements, Applicants have identified a need for improved performance, particularly over a broad temperature range. The “single mode” fiber-touching-the-lens design can also be used with multimode fiber, producing a lower-loss connector because of the elimination of the fiber-to-air Fresnel-loss interfaces.
The prior art expanded beam connectors shown in FIGS. 4(a) and 4(b) involve a clearance fit between the housing 49, 50 and the ferrule 43, 45, respectively. Applicants have determined that this clearance fit is one of the underlying causes of the diminished optical performance of the connectors over a wide temperature range. Specifically, the clearance fit requires tolerance between the housing and the ferrule, which leads to tolerance buildup (e.g., in the range of 0.5 to 2.5 microns.) Even at low temperatures, excess clearance within design limits has been found to be detrimental to performance. As temperatures increase, the housing tends to expand to a greater extent than the ferrule, therefore amplifying the tolerance buildup between the ferrule and the housing. This tolerance buildup coupled with disparate thermal expansion of the housing and ferrule causes an offset and skewing effect of the ferrule within the housing. For example, referring to the connector 30 in FIG. 3, as spring 33 pushes the rear of the ferrule 31 forward, the rear can be pushed to one side of the housing 32 due to the tolerance dt between the ferrule 31 and the housing 32, causing the ferrule to skew (as indicated by the arrows), and either an offset occurs at its endface or a tilt of the ferrule can create an angle between the fiber axis and the lens axis which will result in large insertion loss variations. Thus, at higher temperatures, the skew and offset of the ferrule caused by tolerance buildup and thermal expansion becomes more severe, often to the point of diminishing optical performance below accepted standards.
Although an interference fit between the ferrule and housing would eliminate this tolerance buildup and its negative effects, Applicants recognize that, at some high temperature, the expansion of the housing becomes so great that it pulls the endface of the ferrule 31 away from the lens 35 to the point of compromising the physical contact between the two. Applicants also recognize that this temperature may be within the expected operating conditions of the connector, especially for a fiber/lens contact design as disclosed in FIG. 3.
Therefore, a need exists for a connector design that delivers desired performance over a wide range of operating temperatures. The present invention fulfills this need among others.